1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier or a printer, and more particularly, to an image forming apparatus for forming an image on a sheet by transferring a toner image formed on an image bearing member onto a sheet sent by a registration roller pair.
2. Description of the Related Art
In an image forming apparatus using an electrophotographic process, a visible image borne on a photosensitive member or a transfer member is transferred onto a sheet such as plain paper separated and fed by a separation/feeding part, to thereby obtain a recording image. In the conventional image forming apparatus, a sheet fed from the separation/feeding part is aligned so that a leading edge side thereof is perpendicular to a transport direction thereof by being brought into abutment against a nip portion between a stopped registration roller pair to form a loop. After that, rotation of the registration roller pair is started at a timing that allows a toner image formed on a photosensitive drum to be transferred onto a predetermined position of a sheet.
In recent years, the image forming apparatus is desired to be compatible with various kinds of media and sizes such as basic weights ranging from 52 (g/m2) to 300 (g/m2) (g paper), types ranging from plain paper to coat paper, embossed paper, and intermediates, and sizes ranging from a postcard size to 13×19 (inch). With regard to cardboard (thick sheet), according to the conventional image forming apparatus, a stopped position of a sheet with respect to the nip portion of the registration roller pair varies depending on the thickness of the sheet. This raises a problem that the position of an image on the sheet is caused to change as well. Proposed as a solution thereto is Japanese Patent Application Laid-Open No. 2003-280485 in which an image is formed in a proper position on the sheet by changing a sheet re-feed timing at a registration part or an image write start timing to compensate a displacement amount of the stopped position.
The position of the image formed on the sheet is not formed in the proper position not only because the stopped position of the sheet with respect to the nip portion between the registration roller pair varies depending on the thickness of the sheet but also because of the following reason. That is, there is an image forming apparatus in which the sheet has a trailing side nipped by the separation/feeding part while being transported by the registration roller pair. When the trailing side of the sheet being transported by the registration roller pair is nipped by the separation/feeding part, the nipping of the sheet by the separation/feeding part imposes a load on the registration roller pair during transport. In this case, an actual sheet transport speed differs from a desired sheet transport speed at which the registration roller pair are expected to transport the sheet. If the actual sheet transport speed produced by the registration roller pair differs from the desired sheet transport speed, the image cannot be transferred onto the predetermined position of the sheet, thereby hindering image formation with high precision.
Hereinafter, the above-mentioned problem is exemplarily described in detail. In a recent image forming apparatus for which a higher speed is demanded, a transport speed produced by each of transport roller pairs located downstream of the separation/feeding part is set higher than the transport speed produced at the separation/feeding part, to thereby increase a jam margin by increasing an interval between fed sheets. The registration roller pair and the transport roller pair come to a state of pulling a sheet out of the separation/feeding part under a load imposed by the separation/feeding part. This lowers a transport efficiency (ratio of the actual transport speed to the desired transport speed) exhibited by the registration roller pair and the transport roller pair. In terms of costs, there is an image forming apparatus in which the separation/feeding part driven for the sheet transport is caused to stop after the sheet is received from the separation/feeding part to the transport roller pair located downstream thereof. The registration roller pair and the transport roller pair come to a state of pulling a sheet out of the separation/feeding part under a load imposed by the separation/feeding part, and hence the transport efficiency is degraded. The load imposed by the separation/feeding part also includes a load due to a reverse rotation of a separation roller of the separation/feeding part to which reverse rotation drive is transmitted. The degradation of the transport efficiency due to the load imposed by the separation/feeding part causes a displacement of the image in the position on the sheet particularly during the transport of a cardboard. Among the sheets having a length that may be subjected to a load imposed by the separation/feeding part, the sheet having a larger length in a transport direction receives a larger influence on the displacement of the image in the position on the sheet due to the load imposed by the separation/feeding part.
In the conventional image forming apparatus, in order to solve the above-mentioned problems, a nipping force applied between the registration roller pair is set strong to thereby reduce the degradation of the transport efficiency. This avoids significant degradation of image precision. However, in view of the current compatibility with various kinds of media, the separation/feeding part imposes a heavy load on a thick sheet and a sheet having a surface low in smoothness. In a case of the thick sheet and the sheet having a surface low in smoothness, the transport efficiency exhibited by the registration roller pair is degraded to such an extent as to exert a large influence on the accuracy of the image. In a case where a roller pressure of the registration roller pair is set high so as to prevent the transport efficiency of the registration roller pair from dropping even during the transport of the thick sheet (cardboard), a torque for causing a registration roller to rotate increases and leads to an increase in cost due to upsizing of a motor. Further, in a case where the sheet to be transported is a thin sheet (thin paper), thin coat paper, or an intermediate, an increase in roller pressure leaves an impression on the sheet due to the roller pressure of the registration roller pair.